The Copyright Act CriteriaThe measures against which use of a work may be deemed fair are:

The purpose of use: Commercial, for-profit use is not fair use, while commentary or criticism may be.

The nature of the work: Quoting factual work, such as a history book, for example, may be fair use. Since music is interpretive, it's more likely to be protected.

The amount of use: This is where the 30-second rule is debunked. Small portions of a work may be permitted as fair use, while the entire work is not. However, there are no specific guidelines for infringement.

Market effects: If your use interferes with the marketability of the original work, you'd likely be ruled against in a fair use claim. While it could be argued that a fair use criticism might decrease sales and affect the market value of the original, this criteria looks for fair use becoming a replacement for the original work.

The amount of use: This is where the 30-second rule is debunked. Small portions of a work may be permitted as fair use, while the entire work is not. However, there are no specific guidelines for infringement.